1. Field of the Invention
The present invention relates to a low thermal conductivity ceramic mainly composed of a sialon and a process for producing the same.
2. Description of the Prior Art
A conventional process for producing a ceramic, wherein various oxides including La.sub.2 O.sub.3, ZrO.sub.2, CeO.sub.2 and Dy.sub.2 O.sub.3 are added as an assistant, is disclosed in, for example, Japanese Patent Laid-open Nos. 183169/1986, 201663/1986, 113767/1987 and 20782/1983.
Among the above-described laid-open documents, Japanese Patent Laid-Open No. 183169/1986 discloses the use of a sialon. The process for producing a silicon nitride sinter disclosed in this laid-open document comprises adding a sinter aid to a silicon nitride powder and sintering the mixture. The sinter aid comprises a sialon and a rare earth oxide. The weight ratio of sialon to rare earth oxide is limited to 1:1 to 3:1, and the amount of addition of the sialon in terms of inner percentage is limited to 4 to 20% by weight based on the silicon nitride powder. In other words, the laid-open document describes that, in the production of the silicon nitride sinter, a sialon is used as a sinter aid, a dense sinter having a high strength at a high temperature can be produced by sintering under atmospheric pressure without sintering under pressure and sinters or sintered ceramic bodies can be mass produced at a low cost even in the case of articles having a complicated shape and articles having a large size.
The proposals disclosed in the above-described laid-open documents, however, aim principally at enhancing the strength at a high temperature through the crystallization of the grain boundary phase rather than a lowering in the thermal conductivity of the ceramics.
Sialon is a material comprising elements of Si--Al--O--N and which has a relatively low coefficient of thermal expansion, a high corrosion resistance at a high temperature, such as oxidation resistance, and an excellent heat resistance. In order to lower the thermal conductivity of ceramics, it is considered necessary to impart a certain feature to the fine structure of the ceramic per se even when the same oxides as those described above, such as La.sub.2 O.sub.3 (lanthanum oxide), ZrO.sub.2 (zirconium oxide), CeO.sub.2 (cerium oxide) and Dy.sub.2 O.sub.3 (dysprosium oxide), are used as additives.